1. Field of the Invention
The present invention relates to an apparatus and a method for reproducing information from a recording medium using holography.
2. Description of the Related Art
Recently, standard of Blu-ray Disc using blue-violet semiconductor laser has realized commercialization of optical disc with recording density of approximately 50 GB in the consumer market. The optical disc is highly demanded to have capacity as large as that of the HDD (Hard Disc Drive) with size ranging from 100 GB to 1 TB.
However, new type density growth technology different from the one using short wavelength and objective lens with large NA is required for the purpose of realizing the superdense optical disc.
There has been a trend to study storage technology for next generation, and the holographic recording technology for recording digital information with holography has been focused. The holographic recording technology is intended to record information in the recording medium by overlapping a signal beam which contains page data information that has been two-dimensionally modulated by a spatial light modulator with the reference beam in the recording medium, and causing the recording medium to modulate refraction index therein using the resultant interference pattern.
When exposing the recording medium with the reference beam used for recording to upon reproduction of the information, the hologram recorded in the recording medium functions as the diffraction grating to generate a diffraction beam. The resultant diffraction beam which includes the recorded signal beam and phase information is recovered as the same beam.
The recovered signal beam is two-dimensionally detected at high speeds by a photodetector such as CMOS and CCD. The holographic recording technology allows a single hologram to directly record the two-dimensional information in the optical recording medium, and the recorded information to be reproduced. A plurality of page data may be overwritten on a certain region of the recording medium, thus executing high speed recording/reproduction of information with large capacity.
In case of reproducing the hologram, the angle of the reference beam has to be stringently controlled. The condition for optimum angle of the reference beam does not necessarily coincide with the one at a time of recording owing to the influence of contraction or expansion, or change in the refractive index of the medium resulting from the temperature difference between recording and reproduction.
Japanese Unexamined Patent Publication No. 2009-80906 (Patent Document 1) discloses the following description as abstract, that is, the optical information recording/reproduction apparatus includes an optical system that collects an information beam and a reference beam to a holographic memory recording medium 10 having a hologram recording layer 13 and a diffraction grating 15 disposed in the hologram recording layer 13, and guides a servo laser beam A into the diffraction grating 15 to transmit it; photodetectors 1 and 3 that detect diffracted transmitted beams A1 and A3 diffracted and transmitted by the diffraction grating 15; an actuator 242 that moves the holographic memory recording medium 10; and an adjusting unit 240 that controls the actuator 242 based on intensities of the diffracted transmitted beams A1 and A3 detected by the photodetectors 1 and 3, and adjusts a position or an angle of the holographic memory recording medium 10.